1. Field of the Invention
The present invention relates to an electrowinning method of metals through electrolysis of a metal chloride solution to precipitate metals on the cathode.
2. Description of the Related Art
The electrolytic metal extraction methods include the electrolytic refining process in which metals are precipitated on the cathode by electrolysis, applying a crude metal for the anode and the electrowinning process in which metals in the electrolyte are precipitated on the cathode, applying an anode for electrolysis.
For such electrolyte, a sulphate bath and a chloride bath have been applied. The chloride bath can achieve a lower production cost including power cost, because the chloride bath has a larger electrical conductivity of liquid than the sulphate bath, which leads to a lower electrolytic voltage. Metals which can be extracted by the chloride bath, for example, are nickel, cobalt, zinc and copper.
In the electrowinning method of metals applying an anode for electrolysis in the chloride bath, chlorine gas evolves at the anode. The chlorine generating mechanism is expressed by the following chemical equation.2Cl−→Cl2+2e−The present invention discusses reducing power consumption, focusing on the fact that the power consumption can be lowered by the following equation, if an anode with a low chlorine overvoltage is applied.Effect of Decrease in Power Consumption=Reduced amount of Overvoltage×Current Density×Sum of Electrode Area×Electrolysis Hour
As an anode with a low chlorine overvoltage, a specification applying platinum component is promising. Conventionally, the following anodes for electrolysis by a specification applying platinum component have been reported, which comprises:
an anode having the first coating layer of platinum-iridium oxide mixture, on which the second coating layer by a mixture of 2-50 mass % of manganese oxide containing non-stoichiometric compound, expressed as MnOx (x being 1.5 or more but less than 2.0) and 50-98 mass % of titanium oxide having a rutile structure is provided (Patent Literature 1); an anode having the first coating layer by a mixture of 20-80 mol. % of platinum and 20-80 mol. % of iridium oxide having a rutile structure and the second coating layer by a mixture of 3-15 mol. % of iridium oxide having a rutile structure, 5-25 mol. % of ruthenium oxide and 60-92 mol. % of titanium oxide, these two layers constituting a unit layer (Patent Literature 2); and an anode having the first coating layer by a mixture of 20-80 mol. % of platinum and 20-80 mol. % of iridium oxide having a rutile structure and the second coating layer by a mixture of 3-15 mol. % of iridium oxide having a rutile structure and 5-25 mol. % of ruthenium oxide and 60-92 mol. % of tin oxide, these two layers constituting a unit layer, the anode being provided with a single, or multiple numbers of the unit layer (Patent Literature 3).
However, all of these anodes have been developed for the use of chlor-alkali electrolysis and the effect of decrease in power consumption in metal electrowinning method is not always sufficient. Further improvement is anticipated.